1. Field of the Invention
The present invention relates to an imaging apparatus. Particularly, the present invention relates to an imaging apparatus capable of making focus adjustments more precisely without causing an increase in size of the apparatus.
2. Description of the Related Art
Focus adjustment methods applied to an optical apparatus such as an electronic still camera includes a method for measuring the distance to an object or a focus adjustment state using a focusing sensor provided separately from an image sensor and attaining an in-focus state by moving a photographic lens to an appropriate position according to the state. On the other hand, a contrast auto focus (AF) method that attains an in-focus state by using an image sensor also as a focusing sensor and moving a photographic lens to a position where the contrast of an object image formed by the photographic lens becomes maximum has been devised. The contrast AF method will be described with reference to FIGS. 6, 7A, and 7B. FIG. 6 is a flow chart illustrating a control procedure of the contrast AF method. FIG. 7A is a diagram illustrating changes in contrast of an object image used by the contrast AF method. In FIG. 6, an in-focus direction determination step is processing to determine in which of a closest distance direction and an infinite distance direction to drive a focus lens to attain an in-focus state. In which direction to drive the focus lens to obtain higher contrast can be determined only after actually moving the focus lens and checking the contrast. Thus, the following control method is devised as a method for detecting in which direction of movement of the focus photographic lens a peak of the contrast is found or determining whether the current state is an in-focus state. As illustrated by curves 201 and 203 in FIG. 7A, a so-called wobbling operation in which the focus lens is caused to reciprocate forward and backward along the optical axis by minute amounts is performed to compare the contrast level of an image at each position of the reciprocation. Then, the direction in which the contrast increases is detected (a curve 202 in FIG. 7A) to cause the photographic lens to move in that direction or an in-focus state is detected (a curve 204 in FIG. 7A) to cause the photographic lens to stop.
If a wobbling driving amount is small like a curve 301 in FIG. 7B in such a wobbling operation, a clear difference of contrast levels of images to be compared does not appear (a curve 302 in FIG. 7B) so that there is a possibility that an in-focus direction cannot be determined. Thus, it may cause a clear difference of contrast levels of images to be compared to appear by making the wobbling driving amount larger than a predetermined amount. The predetermined wobbling driving amount changes depending on an aperture value f and, as illustrated by curves 304 and 305 in FIG. 7B, a peak of contrast generally becomes flatter and the contrast changes more slowly with an increasing aperture value f compared to a small aperture value f. Thus, a larger wobbling driving amount (a curve 303 in FIG. 7B) is to be set.
When, as described above, the contrast AF method that uses the image sensor also as a focusing sensor is used, an operator can attain an in-focus state while checking an object image through a video display unit in real time. However, if the wobbling driving amount is increased more than necessary, an occurrence of blurring in the object image unallowable for the operator can be considered so that image fluctuations disturbing to the operator may occur. Blurring unallowable for the operator is more likely to occur when a surface on which an object image is actually formed is outside a depth of focus to a focal plane of a light flux from the object. The depth of focus is a range that allows the operator to regard an object image in an in-focus state when an image formation surface is within the range from the focal plane. The depth of focus exists, if the depth of focus is D, in the range of ±D along the optical axis from the focal plane. If the aperture value of the photographic lens is f, the depth of focus D is calculated by using a formula of D=f*δ, where δ is the diameter of the allowable circle of confusion and empirically takes a value of P≦δ≦4·P for an electronic still camera if the pixel pitch of an image sensor is P. Thus, to reduce an occurrence of image fluctuations disturbing to the operator, the wobbling driving amount can be set in such away that the image formation surface is within the depth of focus ±D from the focal plane in an in-focus state. In view of the above circumstances, the wobbling driving amount to determine the in-focus direction or to determine whether in an in-focus state is set as k·D=k·f·δ. k is a constant of proportionality and may be set to ¼ to ¾ empirically.
On the other hand, Japanese Patent Application Laid-Open No. 2005-148610 discusses an apparatus that improves the speed of a focusing operation, when compared with the method for wobbling the photographic lens, by wobbling an image sensor within a limited range during the focusing operation and controlling the operation of the photographic lens based on a sharpness degree (state of blurring) of an image during the wobbling.
If the aperture value f is large in an apparatus according to Japanese Patent Application Laid-Open No. 2005-148610, just like in the case of wobbling a photographic lens, the wobbling driving amount for the image sensor for determining the in-focus direction increases. In such a case, since the image sensor reciprocates a longer stroke in the apparatus, a space for the stroke in the apparatus is to be provided, thus leading to an increase in size of the apparatus.